Why use purge air? And what practicalities to consider.


Courtesy of AMETEK Land

Many AMETEK Land instruments use purge air, in the post I am going to explain the purpose and some of the practicalities surrounding this technology.

In general, purge air is used on optical instruments such as emissions monitors and pyrometers which are mounted on industrial processes. It is important to keep the optical surfaces of such instruments clean, as contamination can affect their performance. An opacity monitor will read high if there is dust on the lens or retroreflector. A pyrometer will report an incorrect temperature if contamination prevents some infrared radiation from reaching the detector. Even more importantly, delicate optics can be damaged by hot furnace gases or corrosive stack gases. Such damage is expensive - in extreme cases, it can destroy the instrument in question.

The important considerations for purge air are reliability, cleanliness and cost. Sometimes, it is possible to make trade-offs. For example, a gas-filter correlation gas monitor such as the Model 9100 can tolerate some contamination of its windows, so a simple purge system may be suitable. Some users have reported successful operation on a negative-pressure process with a passive purge – simply leaving the purge inlet open to atmosphere and allowing the stack draft to pull ambient air through the purge body. Although the low cost makes this attractive, it is also unreliable – even a brief period of positive pressure in the duct can cause severe damage to the instrument – and so we do not recommend this approach.

In most cases, a side-channel blower with large-area filter is the best solution. It requires no services apart from mains power, and it is reliable, having only one moving part, and service lifetime in excess of 10 years is common. Maintenance is limited to replacing the air filter element from time to time. A Y-piece fitted to the outlet allows a single blower to supply more than one purge, ideal for a cross-stack instrument such as an opacity monitor.

It surprises many people to learn that compressed air is not a good choice for air purge in most applications. Although it is commonly-available on an industrial site, the volume of air needed makes it impractical to use as the primary purge source. For example, a 4500 MkIII opacity monitor requires a total of 35 cfm (61 m3/hour). Although compressed air is often thought of as a free resource, running and servicing a compressor is costly so the typical payback time for a side-channel blower purge system is in the region of 6 months. In addition, the quality of compressed air can vary - it is important that air used to purge an optical instrument is dry and free of both oil and particles.

For those applications where a side-channel blower is impractical, AMETEK Land offers a filtered air-mover, also referred to as an eductor. This uses a small volume of clean compressed air to induce a much larger flow of ambient air through a filter and into the instrument’s purge chamber. The educator reduces the purge air consumption by a factor of five, bringing the required volume down to a manageable level in most cases. The air-mover works well in negative- or neutral-pressure processes but it is not suitable for positive pressure applications.

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